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Chemical vapour deposition of germanium-containing films by IR laser-induced decomposition of ethoxy(trimethyl)germane.

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APPI-IED ORGANOMETALLIC CHEMISTRY, VOL. 9, 667-673 (1995)
Chemical Vapour Deposition of
Germanium-containing Films by IR
Laser-induced Decomposition of
Ethoxy(trimethy1)germane
Radek Fajgar,* Zdengk Bastl,t Jaroslav TIaskalS and Josef Pola*§
Academy of Sciences of the Czech Republic, * Institute of Chemical Process Fundamentals, 165 02
Prague 6-Suchdol, Rozvojova 135, t J. Heyrovsky Institute of Physical Chemistry, 182 23 Prague 8,
and 3 Institute of Inorganic Chemistry, 25 068 Re?, near Prague, Czech Republic
Carbon Dioxide (CO, ) laser-induced decomposition of ethoxy(trimethy1)germane(ETG) results
in a substantial stripping of organic substituents
from germanium and leads to deposition of organogermanium films, the composition of which is
dependent on the mode of laser irradiation. Direct
absorption of laser radiation in ETG affords material rich in Germanium, while a sulfur hexafluoride (SF,)-photosensitized process produces a
deposit composed of Germanium, Carbon,
Hydrogen and Oxygen. The deposited materials
can be modified by chemical reactions with acetic
anhydride and atmospheric moisture.
Keywords: chemical vapour deposition; laserinduced decomposition; organogermaniurn film;
ethoxy(trirnethy1)germane
INTRODUCTION
The preparation of polymeric organogermanium
oxides, analogous to silicones, has long been of
interest in organogermanium chemistry, but traditional methods of preparing these organogermanium polymers using hydrolysis of
dihalogermanes,1-3 dialkoxygermanes,4-6 organogermanium
trichloridesG9
or
tetraalkoxygermanes" have been recognized to yield
only low-molecular-weight, water-soluble oligomers, germanium dioxide or digermoxanes. An
alternative approach to the production of polymeric organo-oxogermanes is the radical
decomposition of alkyl(a1koxy)germanes in the
gas phase. Previous studies on metallo-organic
chemical vapour deposition (MOCVD) reveal
§
Author to whom correspondence should be addressed.
CCC 0268-2605/95/080667-07
01995 by John Wiley & Sons, Ltd.
that pyrolytic" and plasma-induced" decomposition of tetraethoxygermane affords only germanium dioxide, but laser-induced decomposition of
tetramethoxygermane yields films of reactive
organo-oxogermanium polymers. l3
These different
reflect the commonly
shared view that the nature of deposited materials
is affected by the structure of the gaseous precursor and by the conditions of precursor
decomposition; this has been well documented
even for laser-induced
MOCVD using
organogermanes. 13-"
In a continuation of our previous studies on IR
laser-induced
MOCVD
of
polyorganooxogermanes from tetrameth~xygermane'~
and of
germanium from tetramethylgermane," we
report in this paper the gas-phase carbon dioxide
(CO,) laser-induced decomposition of ethoxy( trimethy1)germane (ETG) and assess the use of
direct infrared multiphoton decomposition
(IRMPD) as well as sulphur hexafluoride (SF6)
photosensitized decomposition (PSD) of ETG as
a technique for preparation (deposition) of polymeric organo-oxogermanes.
EXPERIMENTAL
Laser irradiation experiments were performed on
gaseous samples of ETG and ETG-SF, compounds contained in a cylindrical glass reactor
(10 cm x 3.6 cm i.d.), equipped with sodium
chloride (NaCl) windows, a PTFE stopcock and a
sleeve with rubber septum. A grating-tuned transversely excited atmospheric (TEA) C 0 2 laser"
was operated at a repetition frequency of 1 Hz
(energy in pulse 0.28 J cm-') on the R(12) line of
the 00"1-02"0 transition (1073.2 cm-') to achieve
absorption in ETG. A grating-tuned continuousReceived 21 September 1994
Accepted 27 February I995
668
wave COz lase?" (output 10 W) operating on the
transition
P(20) line of the 00'1-02"O
(944.2 cm-') was chosen for the irradiation of the
ETG-SF, mixture when absorption in SF,, was
effective. The laser beam of the pulsed and continuous radiation was focused at the centre of the
horizontally positioned reactor using a Ge or
NaCl lens, and the substrate (NaCI, potassium
bromide (KBr) glass or aluminium) for the
deposit was housed 1.5 cm beneath the focal
point. The laser beam energy was measured with
a laser energy pyroceramic sensor (Charles
University, ml-1JU model) or a Coherent Model
201 power meter, and the laser line used for the
irradiation was verified with a model 16-A spectrum analyser (Optical Engineering Co.).
The samples for laser irradiation were prepared
by a standard vacuum-line technique and the
pressure of ETG was measured by a Barocel
pressure transducer (model 1570). The IR spectra
before and after irradiation were recorded with a
Specord 75 model (Zeiss) IR spectrometer. The
depletion of ETG was monitored at 810 and
1040 cm-'. Gaseous products of the laser-induced
decomposition of ETG were identified by their
absorption spectra (methane 1300 cm-', ethene
950 cm-', ethyne 730 cm-' and acetaldehyde
1746cm-'), by their mass fragmentation and by
their retention times. For the latter purpose,
helium was expanded into the reactor to atmospheric pressure and gaseous samples were
injected into a G U M S (Shimadzu Q P 1000)
quadrupole mass spectrometer (column 1.2 m
long packed with Porapak P, programmed temperature 25-160 "C). The amounts of gaseous
compounds were determined by using the absorptivity of the diagnostic (strong, non-overlapping)
bands and the comparison with molar absorptivities measured with authentic samples (cm-I,
lO'XkPa-'cm-'): ETG, 1040, 64 and 810, 65;
CH,, 1300, 30; C2HZ,730, 94: C2H4, 950, 70;
CH,CHO, 1746,58.
The deposit on glass or NaCl substrates was
investigated by means of X-ray photoelectron
spectroscopy, scanning electron microscopy,
UV/Vis and IR spectroscopy.
ESCA measurements were made using a VG
ESCA 3 MkII electron spectrometer. The pressure of residual gases during accumulation of the
spectra was in the 10-6Pa range. The measurements were performed using AIK, (1486.6 eV)
radiation. The spectrometer was operated in the
fixed-analyser transmission mode with a pass
energy of 20 eV giving a resolution of 1.1eV on
R. FAJGAR, Z . BASTL, J . TLASKAL AND J . POLA
the Au f7,2 line. The preparation chamber of the
spectrometer was equipped with a cold cathode
ion gun.
The spectra of Ge 2p, 3d, i" 1s and 0 1s
photoelectrons and Ge L3M45M,5Auger electrons
were measured. The ratios of atomic concentrations were determined by correcting the photoelectron peak areas for their cro$s-sections*' and
by taking into account the dependence of the
photoelectron mean free path and analyser transmission on electronic kinetic
The
overlapping spectral features we re resolved into
individual components of GausGan-Lorentzian
shape using a modified version of the damped
non-linear squares procedure published by
Hughes and Sexton.24For binding energy data we
estimated the error limit of +0.2eV. The estimated accuracy of the calculated ratios of atomic
concentrations amounted to k 10'%.
UV/Vis absorption spectra of the deposit were
measured, using a Hewlett-E'ackard 8451A
spectrometer, in the range 120-900 nm.
Scanning electron microscopy (ISEM) studies of
the deposit were performed O I I an ultra-high
vacuum Tesla BS 350 instrument equipped with
an energy-dispersive analyser of !<-ray radiation,
Edax 9100/65. An ECON detectcx in the shield
mode (plastic window) was used for qualitative
determination of light elements. The morphology
of the samples was investigated mostly using an
accelerating voltage of 4 kV.
ETG samples as well as authentic samples of
trimethylgermane and tetramethj,,lgermane were
prepared as reported'" and distilled under vacuum
before use. Sulphur hexafluoride was a commercial sample from Fluka.
''
RESULTS AND DISCUSSION
Infrared multiphoton decomposition
(IRMPD)
Focused irradiation by the C 0 2 laser (0.28 J in
pulse) in the strong absorption band ( v ~mode)
~ - ~
of ETG at 1073 cm-' results (Figs 1 and 2) in the
depletion of ETG and formation of methane,
ethene, ethyne, acetaldehyde, carbon monoxide
and a compound whose structure was tentatively
assigned to (H2CH3Ge)?0[infrared absorption at
2035 cm-'; mass spectrum - CH4(CH3)2Ge20+
(characteristic mass range 190-202)]. A significant amount of a solid brown material, deposited
CVD OF GERMANIUM-CONTAINING FILMS
669
II
u1
0.4.
500
0
iooo
Number of pulses
Figure3 Distribution of volatile products in IRMPD of
ETG: B,ETG; A , C2H4; 0, CZHZ; 0, CH3CHO; 0, CHI).
-d
I
1000
2000
3000
Wavenumber, cm-1
Figure 1 IR spectra of the irradiated (900 pulses, (a)] and
initial (b) ETG, and of the solid deposit (c) and the deposit
treated with acetic anhydride (d).
all over the inside of the reactor, is produced
concomitantly. Mass balance measurement (IR
spectral determination of amounts of gaseous
products) (Fig. 3) is consistent with approximately 80% of carbon being used for the formation
of the gaseous products. A very weak absorption
band at 2035 cm-' and the high molar absorptivity
of
organogermanium
hydrides
at
this
~ a v e l e n g t h lead
' ~ us to believe that the volatile
digermoxane is formed only at pressures lower
than 0.01 kPa, which implies that almost all the
germanium, less than 20% of the carbon and less
than 60% of the oxygen from the ETG is incorporated in the deposited material. This estimation
is consistent with the stoichionietry of the deposit
as determined by X-ray photoelectron spectroscopy (XPS) analysis (Table 1).
Photosensitizeddecomposition (PSD)
Focused irradiation by the continuous-wave (cw)
CO, laser on ETG-SF, (each component 0.7 kPa)
mixtures (incident energy 40 W cm-') tuned to
Table 1 XPS core level binding energies, Auger parameters
(eV) and composition of the deposits.
Source of
deposit
IRMPD
IRMPD~
PSD
PSDb
IRMPD/Ac,O
Auger
Ge 2pyz Ge 3d parameter'
1219.3
1217.9
1220.3
1218.1
1218.2
1220.3
IRMPDIAcZOd 1217.3
1219.3
1221.0
1217.2
1220.4
Retention time
Figure2 Typical GC-MS trace of the irradiated ETG (a)
and ETG-SF, (b). Peak identification: 1, air; CO; 2, CHI; 3,
C2H4,C,H,; 4, CH3CHO; 5, ETG, C,H,OH; 6, (CH,),Ge,
(CH,),GeH, ETG, C,H,OH; 7, (CH,H2Ge),0.
-
1174.3
1174.7
1171.4
1174.5
-'
Gel
Gel oC,300
Gel oC4,OI
Gel ,C1 8008
Gel o c 0 0 0 1 o
-'
Gel o C U 4 0 1 o
1174.7
1170.9
Clean Gee
GeO?'
-
29.6
33.3
Value of Auger parameter based on Ge 3d peak.
After 10 min of sputtering with argon ions (energy 5 keV, ion
current approx. 40 PA).
'Because of overlap of Auger peaks which could not be
separated, the Auger parameter values were not calculated.
After 1min of sputtering with argon ions (energy 3.5 keV,
ion current 40 PA).
Reference (commercial) samples of polycrystalline germanium and GeO, respectively.
a
1
30.3
29.9
32.4
30.1
29.9
32.3
Stoichiometry
R FAJGAR, Z. BASTL. J TLASKAL AND J . POLA
670
the strong absorption band of SF, at 944 cm-' has
a similar effect to direct absorption of the laser
radiation in ETG. SF, acts as an energyconveying agent" '' and induces homogeneous
decomposition of ETG. Irradiation times shorter
than 40 s were sufficient to achieve approximately
60% decomposition. Gaseous products formed
included those observed upon IRMPD of ETG
and also tetramethylgermane and trimethylgermane (Fig. 2b). Formation of a brown
powder-like material was observed as well, but its
amount was smaller than in IRMPD.
The similarity of the main volatile products in
IRMPD and particle-size determination (PSD) of
ETG indicates that the major steps contributing
to the decomposition are the same. The analysis
of the deposited materials by XPS (Table l), as
well as the amounts of gaseous products (Fig. 3)
are in line with a substantial cleavage of organic
moieties from germanium. The cleavage of the
Ge-0 and Ge-C bonds is easier than that of the
0 - C bond (homolytic bond dissociation energy
300 kJ mol-'," 240 kJ mol-',") and 340 kJ mol-',
respectively). The initial Ge-C and Ge-0 cleavages in ETG are obviously followed by radical
reactions with parent ETG as hydrogen abstraction from the OCH,CH, (major, Eqn [2]) or
OCH,CH, (minor, Eqn [ 3 ] )units, loss of acetaldehyde (major, Eqn [4]), or ethene (minor Eqn
[ S ] ) . These products can also be formed by direct
four-centre /?-elimination reactions (Eqns [6],
[7]). Second-order radical reactions, e.g. disprois less likportionation of an ethoxy
ely. Decomposition of acetaldehyde3' (Eqn 181)
and ethanol' (Eqn [9]) can occur, too. The proposed steps are given in Eqns [ 1]-[9].
*
I
I
Ge--OCH,CH,
+ CH;
+ 'OCH,CH3
CH,'
CH;
I
t E T G - + CH,-Ge'
+ (CH,),GeOCH,CH,-+CH,
+ (CH ,),GeOCH'CH,
+ (CH,),GeOCH,CH,-.CH,
+ (CH,) ,GeOCH2CH,'
I
[11
PI
(31
+ CH,CHO [4]
(CH,),GeOCH2CH2'-t (CH,),GeO' + C2H4 [5]
(CH,),GeOCH'CH,-+ (CH,),Ge'
(CH3)3GeOCH2CHI+(CH3),GeH+ CH,CHO
[61
(CH7)7GeOCH,CH3-+
(CH,)Ge( )H + C,H,
CH,CHO+ CH,
+ CO
[7]
PI
CH,CH,OH+ C2H4 + H,O
[91
Formation of trimethylgermarre might also
occur via molecular expulsion of ethene oxide
which rearranges" 3 3 into acetaldehyde Eqn [lo]),
rather than by an ~nlikely~"'~
hydrogen abstraction by the (CH,),Gc' radical.
'H- C H ~
CH~CHO
Properties of the deposit
ESCA, FTIR and SEM analyses of the deposits
afforded by IRMPD and PSD of ETG reveal that
the materials contain germanium, carbon, hydrogen and oxygen and that they have different
properties depending on the way they were produced. Energy dispersive X-ray spectrometry
(EDX)-SEM analysis of the bulk material (of ca
0.3 pm thickness) is in line with greater amounts
of the deposited agglomerates and with a higher
proportion of germanium in the deposit from
IRMPD compared with that from PSD (Fig. 4).
These results conform with the stoichiometry of
superficial (up to 5 nm) layers analysed by ESCA
(Table 1).The deposit from PSD contains neither
fluorine nor sulphur, which shows no chemical
involvement of the sensitizer. The analysis of the
layers beneath those removed by ion sputtering
reveals that the material deposited by IRMPD is
very similar to elemental germanium, while that
obtained by PSD contains, apart from germanium, carbon and oxygen also. The amounts of
oxygen in superficial layers of both deposits is
higher than it would be if it corresponded to the
observed Ge-0 cleavage/formation of acetaldehyde and ethanol, indicating that superficial
layers incorporate oxygen from the atmosphere.
The Auger parameters for the unchanged and
ion-sputtered
deposits
show
that
the
IRMPD-originated material contains elemental
germanium, while that obtained from PSD incor-
67 1
CVD OF GERMANIUM-CONTAINING FILMS
b
a
Figure 4 SEM image and EDX-SEM trace of the deposit afforded by PSD (a) and IRMPD (b).
porates superficial GeO, , elemental germanium
being incorporated only in deeper layers.
Optical absorption data of the film obtained by
IRMPD (Fig. 5 ) show that the absorption edge
(transparency) is not reached at wavelengths
lower than 800 nm.
The deposits do not possess good adhesion to
glass, sodium chloride or aluminium. They are
soluble in acetone and tetrahydrofuran to form
brown solutions, but are insoluble in ethanol and
alkanes.
The deposits obtained from IRMPD and PSD
exert a similar pattern of infrared absorption
bands (770, 850, 1230, 1370-1400, 2900 and
.-0
+
.
3
2970 cm-'; Fig. lc); this is c ~ n s i s t e n t ~ 'with
. ~ * the
occurrence of Ge-0-C
and C-H bonds. In
order to prove that the deposits contain
Ge-OC2Hs groups, we examined the reactivity
of the layers with water vapour and acetic anhydride. It is known that water reacts" with alkoxygermanes to yield GeOz and ethanol, and that
acetic anhydride reacts (R. Fajgar, unpublished
results) with ETG giving ethyl acetate and
acetoxy(trimethy1)germane. We found that the
introduction of air into the evacuated reactor
containing a deposit from IRMPD leads to formation of ethanol (new bands at 1055, 1230 and
1400cm-' and a broad band centred at
3400cm-'), and that treatment of the same
deposit with acetic anhydride vapour (exposure to
acetic anhydride, evacuation and exposure to air)
alters the infrared spectrum of the deposit due to
the occurrence of a stronger band at 1400cm-'
and a new band at 1600 cm-l (Fig. Id) which can
U
P,
L
0
I
2
<
+Ge-OCn2CH3-
I
-
-
J
A
d
-
I
15 D
-
Figure 5 Absorption spectrum of the deposit from IRMPD.
I
+Ge-OH+
GH~OH
I
H20
+Ge-OC(O)CH3+Ge-OH+
c: 2 0
Wavelength, nm
H20
-CH3COOC2Hj
I
Scheme 1
1
I
CH,CO*H
R. FAJGAR, Z . BASTL. J. TLASKAL AND J . POLA
672
I
I
I
I
I
I
1210
1220
1230
BINDING ENERGY (eV)
(a)
280
285
290
BINDING ENERGY (eV)
(b)
-
l
"
"
I
'
"
'
I
'
"
'
be assigned to adsorbed acetic acid. These
changes appear to be due to the reactions shown
in Scheme 1 and they reveal that the deposited
materials can be chemically modified.
The incorporation of oxygen into the deposit
from IRMPD has also been confirmed by XPS
analysis (Table 1); the equal amounts of germanium and oxygen in beth superficisl and deeper
layers of the treated deposit show that the incor'poration of oxygen can be enhanced by chemical
treatment, and that the materials perhaps have a
porous structure facilitating the penetration of
reactive vapours. The approximately ten-fold
increase in the oxygen content in deeper layers of
the deposit from IRMPD experimerits upon treatment with acetic anhydride indicates that the
incorporation of oxygen cannot be solely due to
reactions of Ge-OC2H, groups, m d that it is
perhaps caused by reactions of reactive (naked)
germanium centres. The nature of tnese reactions
as well as that of the centers are at present
unknown. The shape of the germantum core level
spectra measured after sample treatment with
acetic anhydride suggests they are composed of
two peaks (Fig. 6a; Table 1) corresponding to the
order of increasing binding energy likely to be
ascribed to Ge-C and Ge-0 and/or Ge-OH
bonds, respectively. After ion sputtering, the
additional peak corresponding to elemental germanium appears in the G e 2p3,* photoelectron
spectrum. This assignment is consistent with C 1s
and 0 1s spectra (Fig. 6b and c, respectively).
Acknowledgement The authors thank the Grant Agency of
the Academy of Sciences of the Czech Republic for financial
support (grant no. 47266).
I
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525
530
535
540
BINDING ENERGY (eV)
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Figure6 Spectra of Ge 2p,, (a), C 1s (b), and 0 1s (c)
photoelectrons of the deposit treated with acetic anhydride
before ( 1 ) and after (2) argon ion sputtering.
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decompositions, trimethyl, german, containing, chemical, induced, deposition, films, ethoxy, germanium, vapour, laser
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